Medical imaging apparatus

ABSTRACT

A medical imaging apparatus for imaging subcutaneous body temperature that comprises a detector ( 22 ) for sensing millimetre wave electromagnetic radiation and a collector for collecting radiation emitted from Scanned spot a patient&#39;s body and directing it along a collection path ( 20 ) to the detector. The collector is configured so that the collected radiation has a defined sensitivity profile across and along substantially the entire length of that path. The collected radiation may have a Gaussian or a Bessel sensitivity profile.

This invention relates to a medical imaging apparatus for imagingsubcutaneous temperature within a body.

Medical imaging using infrared imaging or thermography to obtain passiveand non-invasive measurements of human body temperature is anestablished technique. This technique is, however, relatively limited,because infrared imaging only effectively measures the surfacetemperature of the body. This is because infrared radiation does notpenetrate body tissue very well, and it is difficult to ascertainsub-surface temperature distributions accurately from such surfacetemperature measurements.

Microwave thermography is often used where tissue temperature at depthwithin bodies is to be measured, see for example the article “MicrowaveRadiometric Imaging at 3 GHz for the Exploration of Breast Tumors” byBocquet et al, IEEE Transactions on Microwave Theory and Techniques, Vol38, No. 6, June 1990. Typically, microwave thermography is done using acontact-probe radiometer operating at a frequency of around 2-3 GHz.Since microwaves can travel further through body tissues, microwavethermography can achieve measurements to a depth of several centimetres.However, whilst temperature contributions are detectable at depth,spatial resolution is generally poor. This is because of the relativelylong wavelengths.

U.S. Pat. No. 4,407,292 discloses another imaging technique. In this,thermal radiation emitted by hyperthermic tumerous tissues is collected,focussed and detected within several frequency bands from 8 GHz to 36GHz. This is done using a lightweight elliptical reflector and abroadband radiometer. A problem with the arrangement of U.S. Pat. No.4,407,292 is, however, that the spatial resolution is poorly defined.Additionally, the image acquisition time is long, due to the relativeinsensitivity of the receiver.

An object of the present invention is to overcome one or more of thedisadvantages associated with the prior art.

According to one aspect of the invention, there is provided a passivemedical imaging apparatus for imaging subcutaneous body temperature, theapparatus comprising a detector for sensing millimetre waveelectromagnetic radiation and a collector for collecting radiationemitted from a patient's body and directing it along a collection pathto the detector in such a manner that the collected radiation has adefined sensitivity profile across and along substantially the entirelength of that path.

By providing a sensitivity profile that is defined along the entirelength of the collection path, improved knowledge of the beam that isincident on the detector is provided. This is because radiation receivedfrom the area on which the device is focused has propagated in awell-controlled and definable pattern. This information can then be usedthrough signal processing to improve the overall spatial resolution ofthe image.

In this context, the sensitivity profile is defined in that its generalform is known along the whole of the collection path. One example ofsuch a general form of profile is a fundamental Gaussian profile.

Preferably, the collector comprises focussing means. The collectorand/or focussing means can be considered to act as an antenna. Thecollector may comprise a feedhorn, in particular a corrugated feedhorn,and a wave guide for supplying radiation to the detector, the feedhornbeing arranged to convert a fundamental Gaussian mode beam of radiation,created by the collector and/or focussing means, into a wave guide modein which radiation propagates through the wave guide to the detector. Inthis way, the feedhorn achieves a fundamental Gaussian mode sensitivityprofile. Alternatively, the apparatus may have a Bessel sensitivityprofile and to that end may include an axicon. This axicon is a cylinderformed with a conical prism at one end.

Preferably, the collector is operable repeatedly to sweep the collectionpath through 360°. To this end, the collector may comprise a deflectorthat is rotatable about one axis to scan the collection path in ascanning direction. Alternatively, the collector may be linearlymovable, so as to provide a raster scan. In either case, the apparatuscan further comprise line-indexing means for moving the collection pathin a direction perpendicular to the scanning direction. The indexingmeans may move the deflector linearly along said axis or may comprisemeans for swinging the deflector about a second axis perpendicular tothe first axis. An advantage of this is that it avoids the need to movethe whole of the imaging apparatus relative to the body in order to scanthe portion of the body to be imaged. Preferably, the apparatus furthercomprises an isolator situated, in use, in the radiation collection pathfor preventing signal leakage from the apparatus into the collectionpath. This feature is useful if the apparatus is used on close rangesubjects. The isolator prevents leakage of radiation from the apparatus,which when reflected back off the target could degrade the sensitivity.

Where the apparatus includes a feedhorn, the isolator may be interposedbetween the feedhorn and the detector or in front of the horn. By doingthe latter, that is placing the isolator between detector and thefeedhorn, it is easier to achieve low insertion loss over a widebandwidth, which is necessary for good thermal sensitivity.

Preferably, the apparatus is operable to form an image from emittedradiation in the frequency range of 10-200 GHz, for example 90-100 GHz.

The apparatus may be sensitive to radiation of a plurality of differentfrequencies. This enables the apparatus to resolve areas of thermalemission in three dimensions.

Preferably, the apparatus includes calibration load means for emittingmillimetre wave radiation at a predetermined intensity, the apparatusbeing operable to direct said radiation to the detector to enable theapparatus to be calibrated. The calibration load may be provided in thescanning path of the imager, so that it is scanned each time the targetis scanned. In this way, the imager can be calibrated for each pass ofthe imager. Where the collection path is rotatable, the load means maybe positioned so as to lie in a line swept by the rotating collectionpath so that the apparatus can be calibrated for each individual sweep.Where the collection path is a raster scan path, the load means may beprovided at one or more ends of the raster scan path, so that the loadis scanned each time a line is scanned. The load means may comprise twoloads and means for maintaining them at different temperatures.Preferably, the calibration load temperatures straddle the range ofsubcutaneous body temperatures to be imaged.

If the detector is linearly polarised, the apparatus preferably includespolarisation means for altering the polarisation of received radiationso as to align with the polarisation of the detector.

According to another aspect of the invention, there is provided anapparatus having a detector that is sensitive to millimetre wavelengthsof electromagnetic radiation; a collector for collecting such radiationemitted from an area of a body and directing it towards the detector,the collector being movable along a collection path and calibrationmeans located in the collection path operable to emit radiation of aknown intensity.

By providing a calibration load in the collection path of the collectorcalibration data can be obtained every time the collector moves alongthe collection path. This means that the imager can be calibrated on aline-by-line basis. This is advantageous.

According to a further aspect of the invention there is provided amedical imaging apparatus for imaging subcutaneous body temperatures,the apparatus comprising a detector sensitive to millimetre waveelectromagnetic radiation and for generating an output representative ofthe image; a collector for collecting radiation from a selected body tobe imaged and directing the radiation to the detector, and an isolatorsituated in the radiation path to the detector and operable to preventinterfering electromagnetic radiation generated by the detector frombeing emitted from the device via the collector means, whilst allowingreceived radiation to reach the detector. The isolation means maycomprise a quasioptical isolator.

In accordance with still another aspect of the invention, there isprovided a medical imaging apparatus for imaging subcutaneous bodytemperatures, the apparatus comprising a detector sensitive to incidentmillimetre wave electromagnetic radiation and for generating an outputrepresentative of the image; a collector for collecting such radiationtravelling from a selected area of a body to be thermally imaged to thecollector along a collection path and directing said radiation onto thedetector means, and a scanner for causing said path to rotate.

By providing a scanner to allow the collection path to be rotated, theselected area of the body or region thereof can be thermally imagedrelatively rapidly.

Preferably, the apparatus includes focussing means for focussing thedetector means on said area, wherein the focussing means is such as togive the apparatus a defined sensitivity profile across and alongsubstantially the entire path length.

Various aspects of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a first medical imaging apparatusincluding a passive millimetre radiometer;

FIG. 2 is a block diagram of a scanner for use in the radiometer of FIG.1;

FIG. 3 is a block diagram of a quasi-optical isolator for use in theimager of FIG. 2;

FIG. 4 is a block diagram of a detector circuit for use in the imager ofFIG. 2;

FIG. 5 is an end view of the imager of FIG. 2 that illustrates scanningof a portion of a patient's body;

FIG. 6 shows a first technique for scanning an area of a patient's body;

FIG. 7 shows a second technique for scanning an area of a patient'sbody;

FIGS. 8-10 illustrate the sensitivity distribution profile and beampattern associated with the apparatus shown in FIG. 2;

FIG. 11 shows visible, infrared and millimetre wave images of a part ofa human hand;

FIG. 12 is a schematic diagram of a modified version of the imager ofFIG. 2;

FIG. 13 is a schematic diagram of another modified version of the imagerof FIG. 2;

FIG. 14 is a schematic diagram of another imager for use in thearrangement of FIG. 1;

FIG. 15 is a schematic diagram of yet another imager for use in thearrangement of FIG. 1;

FIG. 16 is an end view of an imager in which calibration loads areincluded on the radiation collection path;

FIG. 17 is a cross section of a hot calibration load for use in thearrangement of FIG. 16;

FIG. 18 is a plan view of the load of FIG. 17, and

FIG. 19 is a cross section of a cold calibration load for use in thearrangement of FIG. 16.

FIG. 1 shows a passive imager 1 that is operable to detect millimetrewavelength radiation emitted from the body. By passive it is meant thatno radiation is directed onto the patient by the imager. Instead theimager is operable to detect radiation that is naturally emitted fromthe patient's body. The imager is connected to electronic circuitry 2for controlling and supplying electrical power thereto and alsoreceiving image data therefrom. Received data is processed and displayedas an image on a computer 4. The imager 1 is positioned a few tens ofcentimetres directly above a tabletop 6 on which a part of the patientto be imaged is rested, in this case the hand. The components of theimager 1 are contained within a housing 8 that has a lower window (notshown) through which an area of the tabletop 6 can be scanned, in orderto obtain the image. The apparatus scans the area in a succession ofparallel lines, such as lines 10 and 12.

FIG. 2 shows the imager 1 of FIG. 1 in more detail. This comprises aplanar mirror 14 that is rotatably mounted about an axis. 16.Optionally, the mirror 14 may be rotatable about two separate axes (notshown). Connected to the mirror 14 is a motor (not shown), which isoperable to rotate the mirror in the direction indicated by the arrow18. The mirror 14 is in registry with the window in the housing 8 and isprovided to scan an area of the patient and direct millimetre radiationreceived from that area into a main optical path 20 and towards adetector 22. As an example, the radiometer may be a 95 GHz heterodynetotal power radiometer 22.

On the optical path between the mirror 14 and the detector 22 is aquasi-optical isolator 28. This is provided to prevent signals leakingout from the apparatus. Certain types of radiometer, especiallyheterodyne designs, can leak local oscillator (LO) signals out of theinput port of the mixer of the radiometer. This can be coupled out viathe antenna towards the subject/target, which can degrade theperformance of the radiometer by causing fluctuations in itssensitivity. This can be misinterpreted as radiation emitted by thetarget. Providing an isolator 28 avoids this effect.

The isolator 28 can take any suitable form, but a preferred version isshown in FIG. 3. This comprises a diagonal polariser 30, which lies onthe optical path 20. Facing the polariser 30 and on a line 31 that issubstantially perpendicular to the main optical path is a surface 32that is able to absorb radiation of the frequency of interest. This willbe referred to as a “beam dump” 32. Downstream of the diagonal polariserand on the main path 20 is a Faraday rotator 33, after which is anotherpolariser 34, in this case a vertical polariser. Facing this secondpolariser 34 is a corresponding off-axis beam dump 36. The diagonalpolariser 30 is orientated to allow the passage of light with apolarisation at 45° to that of light passed by the vertical polariser34.

The isolator of FIG. 3 acts as a four-port circulator in which two portsare terminated. Electromagnetic radiation of the desired frequencyselected by the apparatus is passed through the isolator. However, anylocal oscillator leakage from the radiometer is sent to the beam dump32. Any signals coming from the dump 32 go to the dump 36 and any straysignal from the dump 36 would go to the main path 20.

Radiation emitted from the isolator 28 is directed into focussing means,for example a high-density polyethylene lens 40 and from there, into afeedhorn, in particular a corrugated feedhorn 42, as shown in FIG. 2.The lens 40 is adapted to focus on a spot on a cylindrical object planefor a given position of the mirror 14 and direct radiation emitted fromthat spot to the feedhorn 42. Radiation focussed by the lens 40 on thefeedhorn 42 takes the form of a substantially fundamental Gaussian modebeam. This has a well-defined profile across and along substantially theentire collection path between the focussing means and the feedhorn 42.The corrugated feedhorn collects this radiation and converts it into awaveguide mode. The received radiation is fed to the detector 22 andused to image the scanned area of the patient's body.

As mentioned above, the sensitivity profile of the radiation collectedin the scanner of FIG. 2 is well-defined. More specifically, thesensitivity profile of the radiation collected is a Guassian profile. Itshould be noted that the feedhorn 42 and/or lens 40 of FIG. 2 can beconsidered to act as an antenna. As a result of the reciprocal nature ofantennas, the sensitivity profile corresponds to the antenna beampattern. This means that were the detector to be replaced with a signalsource or emitter, the apparatus would emit along the collection path abeam having a fundamental mode Gaussian intensity profile.

In the apparatus of FIGS. 1 and 2, scanning of a target area of apatient's body is effected by rotating the mirror 14. This provides asingle line scan. To collect data over a wider area, the housing 8 ismounted on a support (not shown) that facilitates controlled indexingmovement of the housing 8 along a direction perpendicular to thescanning direction, indicated by line III of FIG. 1. Indexing occurs atthe most once for every revolution of the mirror 14. In order to reducethe effects of noise, the system can be arranged to average the resultsof a number of successive scans along each respective line. In thiscase, the mirror undergoes a number of revolutions, for example five, atany given axial position before indexing occurs. This improves thesignal to noise ratio of the device. However, it will be appreciatedthat this is done at the expense of the speed of image acquisition.

FIG. 4 shows an example of a detector or radiometer 22 that can be usedin the imager of FIG. 2. This comprises a mixer 44 for combining areceived signal 5 with a signal from a local oscillator 46. Connected tothe mixer 44 is an IF amplification stage 48 for amplifying and bandpass filtering the intermediate frequency IF signal received from themixer. The output of the IF amplification stage 48 is connected to asquare law detector, for example a diode 50. Connected to the output ofthe diode 50 is an electronic amplification stage 52 that is operable toamplify an incoming signal, integrate it using a low pass filter andamplify the output to give a voltage proportional to the detected power,that is in turn proportional to the brightness temperature of the areabeing imaged.

The measurement of brightness temperature typically has a temperaturesensitivity given by: ΔT=T_(syt)(Bt)^(−1/2), where T_(sys) is the systemnoise temperature, B is the pre-detection bandwidth and t is theintegration time of the measurement. For a radiometer of given noise,temperature and bandwidth, the temperature sensitivity can be improvedby increasing the integration time. This is a trade-off against theimage acquisition time. Typical integration times per pixel might be1-10 ms. This also governs the beam-scanning rate.

The choice of what frequency band to use for the imager depends on anumber of factors and is governed by the dielectric properties of bodytissue and how they vary with frequency. The frequency band is set bythe detector electronics 22 and in particular theoscillator/mixer/filter combination shown in FIG. 4. The mostcomprehensive publications on the dielectric properties of varioustissue types are “The dielectric Properties of Biological Tissues: I.Literature Survey” by Gabriel et al, Phys. Med. Biol., 41, 1996, pp2231-2249; “The dielectric Properties of Biological Tissues: II.Measurements in the Frequency Range 10 GHz to 20 GHz” by Gabriel et al,Phys. Med. Biol., 41, 1996, pp 2251-2269, and “The dielectric Propertiesof Biological Tissues: III. Parametric Models for the DielectricSpectrum of Tissues” by Gabriel et al, Phys. Med. Biol., 41, 1996, pp2271-2293. These cover measurements of up to 20 GHz. Very littlereliable data exists above 20 GHz. Nevertheless, in general a longerwavelength penetrates through more tissue, whereas a shorter wavelengthis desirable for good spatial resolution. Shorter wavelengths arereflected less by the skin reducing complications due to reflection ofthermal energy from the surroundings. By considering the properties ofdifferent tissues, the frequency range for radiometric imaging of thebody temperature is 10-200 GHz. Within that range, the 90-100 GHz bandgives a reasonable compromise between penetration depth and spatialresolution. Target values for penetration depth and spatial resolutionare of the order of a few millimetres.

In use of the apparatus of FIGS. 1 and 2, the mirror 14 is rotated aboutthe axis 16 so that an area of the patient's body can be scanned. Asshown in FIG. 5, when the mirror 14 rotates, the collection path isswept through 360°, and so the scan line 12 is in the form of acircumference swept out by the path. In FIG. 2, the lines 24 and 26indicate a collection path along which millimetre wave electromagneticradiation travels from a spot 25 to be imaged by the apparatus to themirror 14. Received radiation is reflected from the mirror 14 and passedthrough the isolator 28 and then travels through the focussing lens 40and into the corrugated feedhorn 42.

The imager 1 acquires the image of a part of the body by obtaining imagedata from each successive one of a number of areas in a single scanningline and then repeating the process for successive lines therebybuilding up an array of imaged areas. Where the mirror is rotatableabout a single axis, as shown in FIGS. 2 and 6, the array of areas lieson a surface of a notional cylinder, and this correspondingly governsthe plane of the captured image. In this case, the mirror makes a singlesweep, or a plurality of such sweeps, at a given level, say line 3 ofFIG. 6. Then the scanner is moved translationally, so that the next linecan be scanned. In this way, an array 58 of scanned areas is built up.Alternatively, where the mirror 14 is rotatable about two perpendicularaxes, scanning could be performed in two dimensions, as shown in FIG. 7.This approach causes the apparatus to scan a volume that is part of thesurface of a sphere, i.e. curved in two planes. In this case,translational movement of the housing of the imager is not necessary.

For any given position of the mirror 14, the apparatus is focused on arespective spot 60 on the cylindrical surface. The sensitivity of theapparatus to incident radiation across that spot varies, as shown inFIGS. 8 to 10. This sensitivity profile has a fundamental mode Gaussianform. FIG. 8 shows the sensitivity profile at various different pointsalong the beam from a position some way in front of the point z=0, seeprofile 62, to a position somewhere behind, see profile 64. As can beseen, the profile retains its fundamental mode Gaussian form, but thewidth of the peak progressively decreases from the profile 62 to aminimum width at the plane z=0, whilst the peaks behind the plane z=0become progressively broader with increasing distance from the plane.The Gaussian mode is preserved throughout the optical path, and enablesthe width of the collection path to be comparable with the wavelength ofoperation, the profile enabling the effects of diffraction to beanticipated or controlled. The mode of the imager is preservedprincipally because of the configuration of the feedhorn 42.

The radiometer in which the invention is embodied allows features belowthe surface of a patient's body to be imaged. To illustrate this andcompare the effectiveness of the radiometer in which the invention isembodied with existing techniques, FIG. 11 shows three scanned images.The first was taken using visible radiation. As would be expected,visible light is unable to distinguish features below the surface of thepatient's hand. The second image was taken using infrared radiation. Inthis case, a small amount of sub-surface detail can be seen at thepatient's finger tips. The third image was taken using the radiometer ofFIGS. 1 and 2. In this case, thermal variations are clearlydistinguishable. Providing images of this nature is advantageous.

Various modifications to the imager of FIG. 2 are possible. For example,rather than having the isolator 28 of FIG. 2 between the mirror 14 andthe lens 40, it could be located between the feedhorn 42 and theradiometer 22, as shown in FIG. 12. Additionally or alternatively, theapparatus can be modified by the inclusion of an axicon 66, as shownFIG. 13. This is located in the collection path between the mirror 14and the lens 40 and is operable to convert between a Gaussiansensitivity profile and a Bessel sensitivity profile 68, as can be seenfrom the inset to FIG. 13. The Bessel profile has a central peak thatdiffracts less over a given distance compared with a fundamentalGaussian profile of the same width. This may improve the depth of fieldof the apparatus.

FIG. 14 shows another apparatus in which the invention is embodied. Inthis, scanning is effected using a curved, rotatable mirror 70. This ispositioned so as to direct radiation incident on it from a first opticalpath 71 into another, orthogonal path 72. Located on this path 72 are insequence a quasi-optical isolator 73 and another, second curvedfocussing mirror 75. This second mirror 75 is fixed and is positioned tofold radiation incident thereon into another orthogonal path 78. Locatedon this second path 78, is a feedhorn 80, preferably a corrugatedfeedhorn, which is connected to a detector 82. Radiation collected inthe feedhorn 80 is fed to the detector 82, where it can be processed toprovide a suitable image of the scanned area. An advantage of usingcurved mirrors 70 and 75 is that they can be formed of materials thatdissipate less of the received radiation than a lens.

FIG. 15 shows a yet further example of an imager in which the inventionis embodied. In this case, the radiometer 84 is linearly polarised. Thepolarisation of the signals received from the lines scanned by themirror can vary with the angle from which they are received. This may ormay not be a problem depending upon what target is being sensed. Fortargets that are largely unpolarised, such as body tissue, it may notmatter. If, however, it is considered important to have a fixedpolarisation at the target, this can be achieved with the addition oftwo quarter-wave plates.

FIG. 15 shows an imager that is adapted to provide a fixed polarisation.This imager includes a linear polariser 83 that is positioned in thecollection path so as to direct unwanted cross-polarised radiation intoa first beam dump 85 and direct polarised radiation to a rotating mirror86. The input linear polariser 83 can optionally be attached to themirror 86. Alternatively, the input linear polariser 83 could be fixed,in which case it would take the form of a cylinder or any other suitableshape that would be in the line of view of the target. The mirror 86directs radiation received from the polariser 83 along a collection pathto a first quarter wave plate 88, which is attached to the rotatingmirror 86. When the input is linearly polarised, this radiation isconverted by the rotating quarter wave plate 88 to circularly polarisedradiation.

On the collection path 89 after the first quarter wave plate 88 is asecond quarter wave plate 90, which converts the circularly polarisedoutput from the first plate into linearly polarised radiation. Thesecond quarter wave plate 90 is aligned with the polarisation of theradiometer, so that radiation downstream from the second plate 90 ispolarised at an angle that is suitable for reception by the linearlypolarised radiometer 84. Next on the collection path 89 is a linearpolariser, more specifically a vertical polariser 91 that is positionedso as to direct unwanted cross-polarised radiation into a beam dump 93.After the vertical polariser 91 are a lens 92, and a linearly polarisedfeedhorn 94 for feeding radiation to the linearly polarised detector 84.Since the feedhorn 94 is in front of the detector, it is the feedhorn 94that defines the orientation of the polarisation. By providing the firstand second quarter wave plates, the radiation received by the detectoris correctly polarised.

In all of the imagers described above, it is important that calibrationbe carried out. To this end, two angularly spaced calibrations loads 96and 98 may be provided in a part of the scan or the collection path thatdoes not include the target, as shown in FIG. 16. Hence, when thescanning mirror is rotated, the scan line 100 intercepts not only thearea of the patient that is to be scanned, but also the calibrationloads 96 and 98. This means that the calibration loads 96 and 98 aresensed every rotation of the scanning mirror. This leads to a high rateof repetitive calibration, which can be used to reduce the effects ofgain variations in the radiometer that cause sensitivity fluctuations.Also, this line-by-line calibration reduces artefacts in the image, suchas stripes, caused by sensitivity fluctuations.

To perform calibration of the radiometer, it is preferable to use twothermal targets having temperatures above and below the range oftemperatures expected in the real scene. To this end, one of thecalibration loads 96 is a hot load and the other 98 is a cold load 46.Any suitable calibration loads could be used. For accurate radiometriccalibration, it is desirable to have the thermal target filling the beamof the radiometer and a uniform, known temperature over that area. Thetemperature should be constant during the time taken to make thecalibration. Preferred examples of the calibration loads are shown inFIG. 17 to 19.

FIGS. 17 and 18 show a hot calibration load 96. This comprises a heatsink layer 102 carried on which is a thermoelectric heating element 104,for example a Peltier element. By applying an appropriate current to thePeltier element, the load 96 can be heated. On the element 104 is a heatspreader layer 106. This is a thin thermally conducting plate, e.g.metal that evens out any small scale temperature variations on the faceof the Peltier device 104 and provides a relatively uniform temperaturedistribution. On the heat spreader layer 106 is an emissive plate 108,which has sufficient thermal conductance to ensure that its temperaturecan be controlled when in contact with a hot or cold plate. To achieve auniform temperature across the emissive plate 108 with a minimum ofthermal gradient towards the edges and corners, the heat spreader 106preferably has the same surface area as the emissive plate 108, and isas big as or slightly larger than the Peltier device 104.

The material of the emissive plate 108 is chosen to have an emissivity Eclose to unity in the frequency range of operation ensuring that itsbrightness temperature TB is very close to its physical temperatureT_(P), since T_(B)=εT_(P). Having a high emissivity means the materialis also a good absorber in the frequency range of interest. A suitableemissive material could be a solid microwave absorber, rather than aporous structure as is the case for many electromagnetic absorbers, suchas Eccosorb MF-110 provided by Emerson & Cuming. The plate 108 should bethin enough to avoid the setting-up of too great a thermal gradient fromthe back to the front surface when the thermoelectric element isoperated. It is also preferable that the plate has a front surface,which is rough with respect to the wavelength of operation, because thisminimises any specular reflections from the surface. In the presentexample, this is achieved by having an outer surface in which regularpyramids 110 are formed.

As noted before, it is desirable to have a substantially uniform andconstant temperature over the surface of the emissive material. Tomonitor the temperature of the emissive plate a thermometer orthermocouple 112 is provided. This is embedded in the bulk of thematerial of the plate 108 so that any variations in temperature areknown. Optionally, multiple thermometers may be used to monitor spatialvariations in temperature. In any case, connected to the thermometer orthermocouple 112 is temperature measurement circuitry for monitoring thetemperature (not shown). Control circuitry can also be connected to thePeltier element 104, so that in the event that changes in temperatureare detected, a control signal can be sent to alter the current appliedto the Peltier element 104, thereby to cause the temperature to returnto a pre-determined value.

FIG. 19 shows the cold calibration load 98. This is identical to the hotload except the Peltier device is arranged to operate as athermoelectric cooling device for the emissive plate. This can be doneby selecting an appropriate current.

A large difference in temperature between the hot and cold loads 96 and98 is desirable provided that the response of the radiometer behavespredictably over that range. However, the temperature difference istypically limited by practical considerations. For example whenoperating in normal atmospheric conditions, too cold a temperature wouldcause condensation and ice to form on the surface of the emissivematerial, which could alter its apparent brightness temperature. Hence,for a radiometer measuring body temperatures calibration loadtemperatures could be in the range of 5 to 10° C. for the cold load and50 to 60° C. for the hot load.

The emissive plates of the hot and cold loads 96 and 98 respectivelyprovide the thermal targets that are used to calibrate the response ofthe radiometer. When these loads are incorporated into the imagers shownin FIGS. 2 and 12 to 15, their location and temperatures are stored bycontrol software provided in the computer 4. This information is used tocalibrate real measurements of a target area of a patient's body. In useof the imagers, radiation emitted from the loads is detected for eachline of the scan. This means that calibration can be done on aline-by-line basis, thereby making the imager both sensitive to overallsystem changes and accurate. Techniques for calibrating imagers areknown and so will not be described herein in detail.

A skilled person will appreciate that variations of the disclosedarrangements are possible without departing from the invention. Forexample, whilst the imagers described previously in detail each includea rotatable mirror so that the collection path rotates, these couldequally be adapted to provide a raster scan of a target area of apatient. Accordingly, the above description of a specific embodiment ismade by way of example only and not for the purposes of limitation. Itwill be clear to the skilled person that minor modifications may be madewithout significant changes to the operation described.

1. A non-contact passive medical scanning imager for imagingsubcutaneous body temperature comprising: a detector for sensingmillimetre wave electromagnetic radiation; a collector for collectingradiation emitted from a patient and directing that radiation along acollection path to the detector in such a manner that the collectedradiation has a defined sensitivity profile across and alongsubstantially the entire length of that path; scanning means for causinga scan of a target area of the patient, and isolation means in the pathof the collected radiation for preventing signal leakage from thedetector being emitted towards the patient's body, wherein the isolationmeans comprises a quasi-optical isolator.
 2. An imager as claimed inclaim 1, wherein the collector comprises a corrugated feedhorn.
 3. Animager as claimed in claim 1, wherein the collector comprises awaveguide for supplying radiation to the detector.
 4. An imager asclaimed in claim 1, wherein the collector is such that the collectedradiation has a Guassian sensitivity profile.
 5. An imager as claimed inclaim 2, wherein the feedhorn is arranged to convert a fundamentalGaussian mode beam of radiation into a waveguide mode in which radiationpropagates through a wave guide to the detector.
 6. An imager as claimedin claim 1 wherein the collector is such that the collected radiationhas a Bessel sensitivity profile.
 7. An imager as claimed in claim 6including an axicon.
 8. An imager as claimed in claim 1 wherein thecollector includes focussing means.
 9. An imager as claimed in claim 1,wherein the scanning means are operable to repeatedly sweep thecollection path through 360°.
 10. An imager as claimed in claim 9,wherein the scanning means comprise a deflector that is rotatable aboutone axis to scan the collection path in a scanning direction across abody.
 11. An imager as claimed in claim 10 further comprisingline-indexing means for moving the collection path in a directionperpendicular to the scanning direction.
 12. An imager as claimed inclaim 11, wherein the indexing means are operable to move the deflectorlinearly along said axis or comprise means for swinging the deflectorabout a second axis perpendicular to the first axis.
 13. An imager asclaimed in claim 1, wherein the imager is operable to form an image fromemitted radiation in the frequency range of 10-200 GHz, for example90-100 GHz.
 14. An imager as claimed in claim 1, further comprising atleast one calibration load for emitting millimetre wave radiation at apre-determined intensity, the imager being operable to direct saidradiation to the detector to enable the imager to be calibrated.
 15. Animager as claimed in claim 14, wherein the calibration load is providedin the scanning path of the imager, so that the imager can be calibratedfor each pass of the collector.
 16. An imager as claimed in claim 14,wherein two calibration loads are provided, together with means formaintaining them at different temperatures, the temperatures straddlingthe range of subcutaneous body temperatures to be imaged.
 17. An imageras claimed in claim 1 wherein the detector is linearly polarized.
 18. Animager as claimed in claim 17 further including polarization means foraltering the polarization of received radiation so as to align with thepolarization of the detector.